The Kepler spacecraft, NASA's built-in-Boulder planet-hunting mission, has discovered two planets that appear to be ideal places for some form of life to flourish because they are an ideal size and in just the right place near their star.

The distant planets, known as Kepler-62e and Kepler-62f, are fraternal twins, circling the same star, an orange dwarf, and are adjacent to each other -- closer together than Earth and its neighbor Mars.

In the past when astronomers found planets outside our solar system, they hadn't matched all the criteria that would make them right for life. Many planets aren't in the habitable zone -- where it's not too hot and not too cold for liquid water.

And until now, the handful of planets astronomers found in that ideal zone were just too big. Those are likely to be gas balls like Neptune, not suitable for life.

The newly discovered planets are in what astronomers called "the Goldilocks zone," meaning conditions are just right for the possible existence of life in some form.

Boulder's Ball Aerospace & Technologies Corp. developed the Kepler flight system and supports mission operations with the Laboratory for Atmospheric and Space Physics at the University of Colorado. It launched in March 2009.

"Today's news reveals the extent of what a NASA mission can accomplish in such a short amount of time," said John Troeltzsch, Ball Aerospace Kepler program manager.

"We are pleased that the spacecraft continues to perform so well that we're able to capture these amazing results."

Kepler-62e and Kepler-62f are the best candidates for habitable planets that astronomers have found so far, said William Borucki, chief scientist for NASA's Kepler telescope.

"The Kepler spacecraft has certainly turned out to be a rock star of science," John Grunsfeld, associate administrator of the Science Mission Directorate at NASA Headquarters in Washington, said in a NASA news release.

"The discovery of these rocky planets in the habitable zone brings us a bit closer to finding a place like home. It is only a matter of time before we know if the galaxy is home to a multitude of planets like Earth, or if we are a rarity."

The discoveries were published online Thursday in the journal Science.

The planets are described by Borucki as slightly wider than Earth, but not too big. Kepler-62e is a bit toasty, like a Hawaiian world, and Kepler-62f is chillier -- more Alaskan, Borucki said.

The planets are 1,200 light-years away. A light-year is almost 6 trillion miles. They orbit a star that is 7 billion years old -- about 2.5 billion years older than our sun.

"This is the first one where I'm thinking, 'Huh, Kepler-62f really might have life on it,'" said study co-author David Charbonneau, of Harvard. "This is a very important barrier that's been crossed. Why wouldn't it have life?"

Borucki said, "If there's life at all on those planets, it must be very advanced."

The Kepler space telescope, which simultaneously and continuously measures the brightness of more than 150,000 stars, is NASA's first mission capable of detecting Earth-size planets around stars like our sun.

Just two months ago, NASA announced that the Kepler mission had discovered in a separate planetary system the smallest planet ever identified outside our solar system, Kepler-37b.